Until now, metal substrates for industrial use have been subjected to chemical conversion treatment for treating the base surface to improve corrosion resistance and adhesion. However, chemical conversion treatment solutions contain large amounts of various ionic components, as well as large amounts of heavy metal components, such as zinc, nickel, and manganese, to improve the properties of the resulting chemical conversion coating film.
When electrodeposition coating is performed using a cationic electrodeposition coating composition directly after the chemical conversion treatment, it is known that an excess of the chemical conversion treatment solution attached to or deposited on the metal substrate adversely affects the electrodeposition coatability, finished appearance, corrosion resistance, and the like.
Therefore, a typical coating line includes a water-washing step, which requires considerable man-hours and time for degreasing treatment, chemical conversion treatment, first water washing, second water washing, pure-water washing, electrodeposition coating, first water washing, second water washing, pure-water washing, and bake drying, as shown in FIG. 1. Furthermore, collection, filtration, treatment, disposal, etc., of wastewater from the water-washing step also require massive equipment and costs.
Patent Literature (PTL) 1 discloses a method for forming a multilayer coating film that enables process streamlining and space savings. In this method, electrodeposition coating is performed without performing water washing after chemical conversion treatment. PTL 1 discloses that even when a chemical conversion treatment solution is carried as a contaminant into an electrodeposition coating composition in the subsequent step, it is possible to obtain a coated article with excellent finished appearance and excellent corrosion resistance without affecting electrodeposition coatability and coated-film properties. However, when water washing is not performed, and if more than specific amounts of sodium ions and potassium ions are attached to the substrate and/or are carried into the electrodeposition coating composition, it is possible that sufficient corrosion resistance and finished appearance are not achieved.
Patent Literature (PTL) 2 discloses a precoating process. In this process, a substrate is subjected to degreasing, washing, and then, without performing surface conditioning, application of a chemical conversion coating solution. Thereafter, electrodeposition coating is performed without washing with industrial water or drying the resulting substrate. In this process also, it is possible that sufficient corrosion resistance and finished appearance are not achieved, as in PTL 1 above. PTL 2 also discloses that when the chemical conversion treatment solution has a low pH, a water-washing step is required between the chemical conversion step and the electrodeposition coating step to prevent rusting.
Patent Literature (PTL) 3 defines the hydrophobic solvent amount and sodium ion concentration in an electrodeposition coating composition bath to lower the minimum temperature for forming a coating film of an electrodeposition coating composition and to thereby prevent a bridging phenomenon from occurring in narrow gap portions. However, PTL 3 is silent about reducing the concentrations of sodium ions and potassium ions in a chemical conversion treatment solution, and about achieving process streamlining or space savings by omitting the water-washing step. Therefore, if water washing is omitted, sodium ions and potassium ions attached to the substrate during chemical conversion treatment can deteriorate the electrodeposition coatability, as well as the corrosion resistance and finished appearance.
Patent Literature (PTL) 4 discloses a method for performing chemical conversion treatment by dipping treatment using an acidic zinc phosphate aqueous solution. In relation to this, PTL 4 defines the sodium ion concentration in the zinc nitrite aqueous solution used as an accelerator. PTL 4 discloses in paragraph [0016] that when the sodium ion concentration is 10,000 ppm on a weight basis in a chemical conversion treatment tank, an excellent chemical conversion coating film is obtained. However, when sodium ions are contained at a high concentration as above, the water-washing step cannot be omitted, and it is thus difficult to achieve process streamlining and space savings.